1. Field of the Invention
The invention pertains to an apparatus for coupling optical transmission lines and more particularly to apparatus wherein the coupling is accomplished through an active device to provide amplification of an optical signal transferred between the coupled optical waveguides.
2. Description of the Prior Art
In extending optical data link technology to multi-terminal applications, it is necessary to insure that sufficient light energy is coupled from one source terminal of a multiplicity of source terminals to any one of a multiplicity of data receiver terminals. Parameters that define the minimum optical energy required at each receiver terminal include the received signal bandwidth, the desired error rate, the detector efficiency and the noise figures of the detector and the amplifier. Optical power received at a receiver terminal depends upon the optical power generated by the light source at the transmitting terminal, the coupling efficiency between the light source and the transmission medium, the optical waveguide attenuation characteristics and coupling losses, power division, and packing fraction losses incurred at intermediate coupling points. The requirement of a minimum received signal level coupled with the inherent system losses limits the number of receiving terminals that may receive useable optical signals from a transmitting source terminal. This limitation is not improved when LED light sources, which provide relatively low optical output energy, and PIN photodetectors, which exhibit relatively high noise figure, are employed as the optical transmitting and detecting devices respectively.
One alternative to an LED-PIN optical link would be to employ a laser source and an avalanche photodiode detector. However, inexpensive long life laser sources are not yet available and when available may require temperature control to avoid burn out. Similarly, avalanche photo diodes are relatively expensive and also require temperature control. Therefore, for reasons of simplicity, reliability and economy, it is highly desirable to utilize an LED-PIN optical link.
The performance of LED-PIN optical links may be enhanced by utilizing a signal regenerating element as a repeater in a point-to-point optical network. Signal regeneration can readily be obtained by converting an optical signal incident to the repeater, that has been transmitted from a preceding terminal, to an electrical signal equivalent thereto, amplifying this electrical signal and using it to drive a light source which then couples an optical signal, representative of the incident optical signal, to an optical waveguide for transmission to the succeeding terminal. However, optical communication systems consisting of a series of point-to-point links are vulnerable to catastrophic failures that may occur when a repeater, transmitter, or receiver ceases to function
A method for preventing this catastrophic failure, due to the malfunction of a single component, would be to employ a star coupler such as that described in U.S. Pat. No. 3,874,781 and properly inserting a repeater in each optical transmission line emanating therefrom. This requires a multiplicity of repeaters and negates the economy that is sought with the utilization of the LED-PIN link. Therefore, a need exists for providing an optical communications link that utilizes a minimum of signal regenerating elements while maintaining a highly reliable, economical system.